"Female service members have faced the reality of combat, proven their willingness to fight and, yes, to die to defend their fellow Americans," the Pentagon chief told a news conference.

"Not everyone is going to be able to be a combat soldier. But everyone is entitled to a chance," he said before signing a document ending the ban.

President Barack Obama, whose inauguration address on Monday called for opening doors to all Americans, hailed the move as "historic" and "another step toward fulfilling our nation's founding ideals of fairness and equality."

The move highlighted evolving social attitudes and marked yet another sweeping change for the military under Obama, who led a drive to end a prohibition on openly gay troops.

Although some Republican lawmakers oppose the idea, it likely will face little concerted opposition, as Americans have become accustomed to seeing women in uniform and at war.

Panetta unveiled the decision after a months-long review by chiefs of all the armed services who unanimously endorsed a gradual change that would be phased in over the next three years.

Under the decision, the armed services will have until January 2016 to carry out the new policy. Military departments would have to submit detailed plans on implementing the order by May 15, 2013, Panetta said.

The change will apply mainly to the Army and the Marine Corps, as the Air Force and Navy already have lifted most prohibitions on women in combat, allowing them to fly fighter jets and fire weapons on ships.

In 2010, the Navy opted to allow women to serve on submarines.

Commanders began taking a second look at the ban in 2010, a reflection of the changing conditions on the battlefield, as women served on the blurry and ever-shifting frontlines in the wars in Iraq and Afghanistan.

General Martin Dempsey, chairman of the Joint Chiefs of Staff, told the same news conference the move was part of a long-term trend in the military and that he had witnessed changes during his time in Iraq.

He recalled being taken aback when he arrived as a division commander and learned the turret gunner in his armored vehicle was named "Amanda."

"And it's from that point on that I realized something had changed, and it was time to do something about it."

Gender-neutral criteria for combat jobs will be drafted without watering down tough standards for physical strength or other skills, he said.

"The burden used to be that we would say, 'why should a woman serve in a particular specialty?' Now it's, 'why shouldn't a woman serve in a particular specialty?'" Dempsey said.

The four-star general said he and other service chiefs expected women would make it into the elite ranks of special operations forces: "I think we all believe that there will be women who can meet those standards."

A defense official said the change would be incremental, to allow each service to ensure a smooth transition.

"With a change of this magnitude, it may take some time," the official, who spoke on condition of anonymity, told reporters.

Women make up about 14.5 percent of the active duty US military, or about 204,000 service members. And 152 female troops have lost their lives in the wars in Iraq and Afghanistan.

Advocates for changing the policy have argued that denying female troops a chance at ground combat jobs effectively blocks them from attaining top commander posts.

Right-leaning commentators also have questioned whether mothers in uniform should be sent into combat, even if they volunteer.

The Marine Corps recently opened up infantry officer training to women, but the two females who volunteered failed to pass the grueling test, with one suffering an injury.

The policy change will likely define the brief tenure of Panetta, who took over as Pentagon chief in mid-2011 after a stint as director of the CIA, where he presided over the raid that killed Osama bin Laden.